Generally, using existing infrastructure, i.e. established telephone lines, an asymmetric digital subscriber link (ADSL) modem provides analog call services and simultaneously provides high-speed digital services, such as data services and Internet services, without interfering with traditional analog call services.
Referring to FIG. 1, a schematic block diagram of conventional ADSL remote terminal (RT) modems will be discussed. As illustrated, the ADSL RT modem 10 includes an input/output terminal (TP), a transformer 2, a hybrid 3, a line driver (LD) 4, a high-pass filter (HPF) 5, an analog front end (AFE) 6, and a discrete multitone transceiver (DMT) 7.
A signal that is received by the input/output terminal (TP) connected to a twisted pair wire is sent to the hybrid 3 through the transformer 2. The hybrid 3 separates the received signal from a transmitted signal, and transmits the received signal to the high-pass filter 5.
The high-pass filter 5 receives the output signal of the hybrid 3, high-pass filters the signal, and outputs the result to the analog front end 6. The analog front end 6 receives the output signal of the high-pass filter 5, anti-aliases the signal, and converts the result into a digital signal.
The discrete multitone transceiver 7 receives the output signal of the analog front end 6, demodulates the output signal of the analog front end 6 complying with an ADSL standard demodulation method, i.e., the American National Standards Institute (ANSI) standard, and outputs the modulated signal to a host through a predetermined interface.
Furthermore, the digital signal generated in the discrete multitone transceiver 7 is sent to the analog front end 6. The analog front end 6 receives the output signal of the discrete multitone transceiver 7, converts the signal into an analog signal, filters the signal, and outputs the result to the line driver 4.
The line driver 4 amplifies the output signal of the analog front end 6, and outputs the amplified signal to the hybrid 3. The hybrid 3 separates a transmitted signal from a received signal, and outputs the transmitted signal to the transformer 2. The output signal of the transformer 2 is sent to the twisted pair line through the input/output terminal (TP).
The high-pass filter 5 cuts off the output signal of the line driver 4, i.e. the transmitted signal, and a voice signal that is input through the input/output terminal (TP). However, since the passband or cutoff frequency of the high-pass filter 5 is constant regardless of the distance between communications service providers and the ADSL RT modem 10, the data rate of the ADSL RT modem 10 may decrease if the distance between the communications service providers and the ADSL RT modem 10 increases.